1. Field of the Invention
This disclosure relates to structural testing. Particularly, this disclosure relates to techniques for monitoring corrosion of structural elements over time in service.
2. Description of the Related Art
The need to monitor the integrity of structural elements arises in many different applications. For example, it is necessary to monitor the structures of aircraft. The aircraft stay in service for many years and may experience environments that may exceed design limits resulting in different types of failures, e.g., fatigue, fracture, corrosion. Therefore, it is necessary to regularly check the structural integrity of the vehicle as part of any prudent maintenance program. Similarly, other types of structures may also require regular monitoring. Highway structures such as overpasses and bridges must be regularly checked. Some building structures may also require regular testing. Conventional testing techniques such as visual inspection, x-ray, dye penetrant, and electrical field techniques (e.g. eddy current testing, etc.) for testing structural elements have many drawbacks.
Visual inspection of structural members often requires some degree of disassembly of the structure. This adds greatly to the overall testing cost. For example, visual inspection for aircraft structures requires substantial disassembly of structure and removal of installed equipment in order to provide the access needed to view the areas of interest at a distance adequate to detect corrosion visually.
X-Ray testing, under the broader heading of radiographic testing, requires specialized facilities and government licenses. The technique employs the ability of short wavelength electromagnetic radiation to penetrate various materials. Either an X-ray machine or a radioactive source can be used as a source of photons. Because the amount of radiation emerging from the opposite side of an examined material can be detected and measured, variations in the intensity of radiation are used to determine thickness or composition of material and reveal any defects. Due to safety issues, X-ray testing also typically requires a complete work stoppage on all other tasks while the testing is being performed.
Dye penetrant testing is also time consuming and messy. Dye penetrant inspection is used to reveal surface breaking flaws through the bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a surface breaking flaw by capillary action. After a period of time, excess surface penetrant is removed and a developer is applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. The constituent penetrant and developer may and their by-products may be identified as hazardous (HAZMAT), requiring costly disposal means.
Finally, inspection methods using the application of electrical fields (e.g., eddy current testing, etc.) are exceptionally time consuming and difficult to read reliably in this type of application and may require alterations to structure. In typical eddy current testing for example, a circular coil carrying an AC current is placed in close proximity to an electrically conductive specimen to be tested. The alternating current in the coil yields a changing magnetic field, which interacts with the test object and induces eddy currents in it. Variations in the phase and magnitude of these eddy currents can be monitored using a second coil, or by measuring changes to the current flowing in the primary coil. The presence of any flaws or variations in the electrical conductivity or magnetic permeability of the test object, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current. The technique is generally limited to detecting surface breaks or near surface cracking and variations in material composition.
In view of the foregoing, there is a need in the art for apparatuses and methods for efficiently monitoring the integrity of structural elements. In particular, there is a need for such apparatuses and methods to monitor corrosion of structural elements without requiring time-consuming disassembly. There is also a need for such apparatuses to be light weight. And there is further a need for such apparatuses and methods to be inexpensive to implement and use. There is particularly a need for such systems and apparatuses in aircraft applications. These and other needs are met by the present disclosure as detailed hereafter.